Concrete Block Calculator
Enter your wall size and block size to get the number of concrete blocks you need, with the mortar joint and waste built in.
What your result means
Blocks needed is your order count with the 3/8 inch mortar joint and 5% waste built in. CMU is uniform, so waste runs low. Mortar is about one 80 lb bag per 30 blocks, ordered separately, and a structural wall needs rebar and grout in the cores per your local code.
How to use this calculator
- Measure wall length and height.
- Standard CMU block is 8 x 8 x 16 (height x depth x length).
- Use 5% waste (CMU is more uniform than brick, less waste).
- Read block count.
- Calculate mortar separately - about 1 80lb bag per 30 blocks.
The formula
Wall area in square inches is length times height times 144. Add 0.375 to the block's length and height for the mortar joint, multiply for one block's footprint, divide, add waste, and round up.
Worked example
Say you've got a 30 by 8 ft wall in standard 16 by 8 inch block with 5% waste. The wall is 240 sq ft, or 34,560 sq inches; each block with its joint covers about 137. That's 252 base blocks and 265 with waste.
Concrete block quick reference
| Item | Value |
|---|---|
| Standard CMU face | 8 x 16 in |
| Blocks per sq ft of wall | ~1.125 |
| Mortar | 1 bag per ~30 blocks |
| Corner / end | Half block (8 x 8 x 8) |
Tips & gotchas
- Standard CMU is 8 by 8 by 16. Half blocks (8 by 8 by 8) go at corners and ends.
- The 3/8 inch mortar joint is standard and included in the count.
- Order one bag of mortar per 30 standard blocks.
- Reinforce structural walls with rebar in the cores, grouted solid.
- Block is heavy. For a big order, line up delivery and a small crane.
Frequently asked questions
How many concrete blocks per square foot of wall?
About 1.125 blocks per square foot with a standard 8 by 16 inch block face.
What is CMU?
Concrete Masonry Unit, the standard hollow concrete block.
Do block walls need rebar?
Yes for any structural wall over about 4 ft tall. Check your local code for the spacing.
Can I dry-stack concrete blocks?
Only for non-structural decorative work. A structural wall needs mortar.
How much do concrete blocks cost?
$1.50 to $3 a block, or $10 to $20 a square foot for an installed wall.
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